Liquid displacement shuttle system and method

ABSTRACT

A multi-vessel gas storage system and liquid displacement shuttle system that utilizes a liquid-piston shuttled to alternate reservoirs and tank banks to evacuate stored gas or other fluids from storage vessels. Preferably, the gas storage and fluid displacement shuttle system includes multiple pressure storage vessels or tanks arranged in tank banks that are preferably coupled in parallel at one end to a high pressure gas manifold to exhaust the stored gas or other fluids from the vessels and coupled in parallel at another end to separate fluid shuttle circuits. The fluid shuttle circuits include cross-piped fluid fill and drain manifolds that are fluidly linked through interposing reservoirs and pumps. In operation, the stored gas or other fluids are evacuated from the storage vessels by shuttling the volume of displacement liquid between alternating banks of storage tanks and reservoirs with alternating pumps.

FIELD OF THE INVENTION

The invention relates generally to the storage and transport ofcompressed gas and, more particularly, to a fluid displacement shuttlesystem and method that facilitates loading and unloading compressed gasor other fluids in multiple vessel storage systems.

BACKGROUND INFORMATION

The gains in storage capacity from increasing levels of pressure underwhich compressed gasses are held come at the cost of discharging largevolumes in a desired short period of time. Many gas storage systems usemultiple pressure vessels interconnected to manifolds. Discharging froma high pressure storage vessel (at for example 3500 psig) to a receivingterminal (rated for example at 1200 psig) can be accomplished bypressure equalization in a first stage of evacuation. The gas remainingin the container, which is now at the lower terminal pressure level,undergoes a second stage of evacuation by connection to a drawdowncompression or low pressure manifold. As a result, the flow ofcompressed gas transported in multiple assemblies of pressure vessels athigh pressures is frequently subject to a bottleneck of prolongedloading and unloading times.

In seeking to improve the evacuation of the contents of these storagevessels, proposed systems, such as that disclosed in Bishop U.S. Pat.No. 6,655,155, seek to displace the gas under full holding pressureusing a displacement liquid in a manner similar to that used to moveproduct from underground storage caverns. Given the gross volume of allstorage containers, it is possible to purge all interconnected vesselsby simultaneous displacement with an equal volume of liquid. However,such an equal volume would require a large shore mounted supply withrecycle facilities in the case of marine transportation. Such a volumewould be impractical to carry on board a ship and require inordinateamounts of motive power. In response to this problem, Bishop advocates astaged tier displacement system designed into the ship reducing a200,000 bbls initial on board storage need to 50,000 bbls of on boardstorage.

Accordingly, it would be desirable to provide an improved evacuationsystem and method that facilitates the reduction of the amount ofdisplacement liquid used for unloading compressed gas from amulti-vessel storage system and to improve evacuation times bydisplacement of the compressed gas contents in their entirety from thestorage vessels.

SUMMARY

The present invention is directed to a multi-vessel storage and fluid orliquid displacement shuttle system, which utilizes a liquid-pistonshuttled to alternate vessels to evacuate stored product such ascompressed or high pressure gas or other fluids from the storagevessels. In a preferred embodiment, the gas storage and fluid or liquiddisplacement shuttle system includes multiple storage vessels or tanks,or banks of vessels or tanks, that are preferably coupled in parallel atone end to a discharge manifold such as a high pressure gas manifold toexhaust the stored product from the vessels and coupled in parallel atanother end to separate fluid shuttle circuits. The fluid shuttlecircuits include cross-piped fluid fill and drain manifolds that arefluidly linked through interposing reservoirs and pumps. In operation,the stored product is evacuated from the storage vessels by shuttlingthe volume of displacement liquid between alternating banks of storagetanks and reservoirs with alternating pumps. Alternatively, a singlepump and storage (reservoir) system could also be used with a morecomplex control system when a greater storage volume is required

Other systems, methods, features and advantages of the invention will beor will become apparent to one with skill in the art upon examination ofthe following figures and detailed description.

BRIEF DESCRIPTION OF THE FIGURES

The details of the invention, including fabrication, structure andoperation, may be gleaned in part by study of the accompanying figures,in which like reference numerals refer to like parts. The components inthe figures are not necessarily to scale, emphasis instead being placedupon illustrating the principles of the invention. Moreover, allillustrations are intended to convey concepts, where relative sizes,shapes and other detailed attributes may be illustrated schematicallyrather than literally or precisely.

FIG. 1 depicts a multiple pressure vessel storage and fluid displacementshuttle system in accordance with the present invention prior to the gasevacuation cycle.

FIG. 2 depicts the system of the present invention in an initial phaseof the gas evacuation cycle.

FIG. 3 depicts the system of the present invention in a subsequent phaseof the gas evacuation cycle.

FIG. 4 depicts the system of the present invention in a subsequent phaseof the gas evacuation cycle.

FIG. 5 depicts the system of the present invention in a subsequent phaseof the gas evacuation cycle.

FIG. 6 depicts the system of the present invention in a subsequent phaseof the gas evacuation cycle.

DETAILED DESCRIPTION

Turning to the figures, a multi-vessel storage system of the presentinvention is shown to include a liquid displacement shuttle system. Inthe liquid displacement shuttle system, use is made of a liquid-pistonwhich is then shuttled to alternate reservoirs for reuse in the nextstorage vessel or tank in the assembly. In tight spaces, the storagespace saved can be used for stored product, and not displacement liquid.Although the liquid displacement shuttle system is depicted anddiscussed in regard to its use in a compressed or high pressure gasstorage system, it is not restricted to use just with compressed gas,and one of skill in the art would understand the liquid displacementshuttle principal being equally suited to moving compatible storedliquids or fluids.

The present invention reduces the volume of displacement liquid to afraction of a storage system's volume by shuttling the volume ofdisplacement liquid between alternating storage tanks or banks ofstorage tanks and one or more reservoirs using one or more pumps.Multiple sets of shuttle links along the length of a ship could operatesimultaneously, while maintaining a lower level of displacement liquidthan advocated by conventional methods (see, e.g., Bishop). As depictedin the figures, the gas storage and liquid displacement shuttle systempreferably operates with vertical storage vessels or tanks clustered toa common manifold collector valve, but can operate with horizontallydisposed vessels or tanks as well.

In a preferred embodiment, as depicted in FIG. 1, the gas storage andfluid displacement shuttle system includes multiple pressure storagevessels or tanks 40 arranged in tank banks T0, T1, T2, T3, T4, . . . Tnpreferably comprising the same number of equally sized vessels 40 suchthat the gross volume of each tank bank is equal. The vessels 40 arepreferably coupled in parallel adjacent their vessel tops to a dischargeor high pressure gas manifold (HPM) 100 to exhaust stored gas or otherfluids from the vessels 40 and coupled in parallel adjacent their vesselbottoms to separate fluid shuttle circuits. The fluid shuttle circuitsinclude cross-piped fluid fill and drain manifolds 30 a, 30 b, 32 a and32 b that are fluidly linked through interposing reservoirs 36 a and 36b and pumps 38 a and 38 b. As depicted, the vessels 40 are coupledthrough multi-position discharge gas valves 16, 17, 18, 19, 20 and 21 tothe HPM 100, which includes a collector valve 102 with a gas deliveryoutlet 104. As depicted, the vessels 40 in a first set of the evennumbered tank banks T0, T2, and T4 are coupled through multi-positionshuttle fluid valves 10, 12 and 14 to a first fluid drain manifold 30 aand a first fluid fill manifold 32 a while the vessels 40 in a secondset of the odd numbered tank banks T1, T3, and Tn are coupled throughmulti-position shuttle fluid valves 11, 13 and 15 to a second fluiddrain manifold 30 b and a second fluid fill manifold 32 b. The firstdrain manifold 30 a is coupled to a first reservoir 36 a, which in turnis coupled to a first pump 38 a, which in turn is coupled to the secondfill manifold 32 b, thus fluidly linking the first drain manifold 30 ato the second fill manifold 32 b and forming the first fluid shuttlecircuit. Similarly, the second drain manifold 30 b is coupled to asecond reservoir 36 b, which in turn is coupled to a second pump 38 b,which in turn is coupled to the first fill manifold 32 a, thus fluidlylinking the second drain manifold 30 b to the first fill manifold 32 aand forming the second fluid shuttle circuit. Interposing the firstdrain manifold 30 a and first reservoir 36 a, and the second drainmanifold 30 b and second reservoir 36 b, respectively, are first andsecond one-way flow check valves 34 a and 34 b. The multi-positionshuttle fluid valves 10, 11, 12, 13, 14 and 15, first and second fluiddrain manifolds 30 a and 30 b, first and second fluid fill manifolds 32a and 32 b, first and second reservoirs 36 a and 36 b, first and secondpumps 38 a and 38 b, first and second flow check valves 34 a and 34 band a low pressure gas displacement system 200 form the liquiddisplacement shuttle system of the present invention. The low pressuregas displacement system 200 includes a low pressure gas manifold (“LPM”)201, which includes a collector valve 204 and is coupled to the vessels40 through one-way low pressure flow check valves 202.

Preferably, the multi position valves are low pressure gas actuated,control logic valves that may be activated by a stroke count on thepumps or tank level detection depending on physical layout of thesystem. The actuator exhaust gas is preferably routed to gas expansionheaters.

As depicted, the tanks, reservoirs and manifolds are preferablyinterconnected through small diameter piping, tubing or the like. Theone way fluid flow check valves 34 a and 34 b permit displacement liquidfrom the drain manifolds 30 a and 30 b to drain into the reservoirs 36 aand 36 b, but not back into the manifolds 30 a and 30 b from thereservoirs 36 a and 36 b, while the one way gas flow check valves 202permit low pressure blanket gas to fill and equalize pressure inevacuated spaces within the vessels 40, but not back into the LPM 201from the vessels 40. The blanket gas may be methane, ethane, butane,propane and the like, or mixtures thereof as apropriate to the storedproduct.

The mode of operation is described below in conjunction with FIGS. 1through 6. The valve sequence and activity within each tank bank, asdepicted in each figure, are listed in Table 1.

Turning to FIG. 1, the system is depicted in a state prior to the gasdisplacement or evacuation cycle to unload the stored gas, with one tankbank, represented by tank T0, initially filled with a non reactivedisplacement liquid such as saturated natural gas liquid (NGL),compatible solvent gas, and the like, or mixtures thereof. The volume ofdisplacement liquid in tank T0 is preferably substantially equivalent tothe volume of gas stored in each individual tank bank. Preferably, thefluid shuttle circuits are also filled displacement liquid. The spaceabove the displacement liquid in tank T0 is preferably filled with lowpressure gas from the LPM 201. The remaining tanks or vessels 40 in tankbanks T1, T2, T3, T4 . . . Tn are all filled with high pressure gas(“HPG”). (Reference to “tank T0, tank T1, . . . tank Tn” in theremaining discussion will be understood by one skilled in the art to bereferring to the specified tank bank T0, T1, etc. As would be furtherunderstood by one skilled in the art, the tank banks may be configuredto include a single vessel or an equal number of a plurality ofvessels).

As depicted in FIG. 2, the unloading process or gas evacuation cycle isinitialized by rotating the shuttle valve 10 of tank T0 to a firstopened position coupling tank T0 to the first drain manifold 30 a of thefirst fluid shuttle circuit. The displacement liquid drains from tank T0into the first drain manifold 30 a feeding the first reservoir 36 a,which in turn feeds the first pump 38 a, as low pressure gas from theLPM 201 flows through the check valve 202 into tank T0 to fill andequalize pressure in the evacuated space above the displacement liquid.The first pump 38, which is coupled to odd numbered tanks starting withtank T1, pumps displacement liquid into the second fill manifold 32 band on into tank T1 which is coupled to the second fill manifold 32 bwith its shuttle fluid valve 11 rotated to a first opened positionpermitting displacement liquid, which acts as a liquid piston, to pushHPG out of tank T1 through the gas discharge valve 17. The dischargevalve 17 is rotated to an opened position allowing gas to feed into theHPM 100 from where it is delivered through the gas outlet 104 toexpansion and heating facilities or to on shore storage.

Tank T2, which is coupled to the first fill manifold 32 a of the secondfluid shuttle circuit with its shuttle valve 12 rotated to a firstopened position, is shown beginning its HPG evacuation cycle being feddisplacement liquid from the second pump 38 b and second reservoir 36 bthrough the first fill manifold 32 a and its open shuttle valve 12. TheHPG is vented to the HPM 100 through the open discharge valve 18. Asdepicted in FIG. 2 and Table 1, the remaining tanks wait in isolatedmode.

FIG. 3 shows tank T0 emptied of displacement liquid and filled with lowpressure gas. The shuttle fluid valve 10 and gas discharge valve 16 oftank T0 are closed to isolate tank T0 from active operations. Tank T1has been evacuated of HPG, its gas discharge valve 17 is closed and itsshuttle valve 11 has been rotated to a second opened position to coupletank T1 to the second drain manifold 30 b of the second fluid shuttlecircuit to drain displacement liquid into the second drain manifold 30 band on into the second reservoir 36 b as low pressure gas from the LPM201 flows through the check valve 202 into tank T1 to fill and equalizepressure in the evacuated space above the displacement liquid. Thesecond reservoir 36 b feeds the second pump 38 b which in turn feeds thefirst fill manifold 32 a and tank T2 which is coupled to the first fillmanifold 32 a with its shuttle 12 open to a first opened position.Displacement liquid fills tank T2 displacing HPG from tank T2 throughits gas discharge valve 18 which is rotated to an opened positioncoupling tank T2 to the HPM 100.

Tank T3, which is coupled to the second fill manifold 32 a with itsshuttle valve 13 rotated to a first opened position, is shown beginningits HPG evacuation cycle being fed displacement liquid from the firstpump 38 a and first reservoir 36 a through the second fill manifold 32 aand its open shuttle valve 13. The HPG is vented to the HPM 100 throughits open discharge valve 19. As depicted in FIG. 3 and Table 1, theremaining tanks wait in isolated mode.

In FIG. 4, tanks T0 and T1 are shown empty of displacement liquid andwith their valves 10, 11, 16 and 17 closed isolating their low pressuregas contents. Tank t2 is shown nearing drainage completion of itsdisplacement liquid to the first reservoir 36 a with its gas dischargevalve 18 closed and its shuttle valve 12 open to the first drainmanifold 30 a. Tank 3 is shown with its shuttle valve 13 open to thesecond fill manifold 32 b and being fed displacement liquid from thefirst pump 38 a, and in the final stage of venting HPG through its gasdischarge valve 19 to the HPM 100.

Tank T4, which is coupled to the first fill manifold 32 a with itsshuttle valve 14 rotated to a first opened position, is shown beginningits HPG evacuation cycle being fed displacement liquid from the secondpump 38 b and second reservoir 36 b through the first fill manifold 32 aand its open shuttle valve 14. The HPG is vented to the HPM 100 throughits open discharge valve 20. As depicted in FIG. 4 and Table 1, theremaining tanks wait in isolated mode.

Turning to FIG. 5, tanks T0, T1 and T2 are shown in displacement liquidemptied mode with their valves 10, 11, 12 16, 17 and 18 set in isolationpositions isolating the low pressure gas contents of T0, T1 and T2. TankT3, which has switched to fluid drain mode, is shown with its gasdischarge valve 19 closed and its shuttle valve 13 opened to the seconddrain manifold 30 b to drain displacement liquid into the secondreservoir 36 b. Tank T4 is shown approaching the final phase ofdisplacing HPG into the HPM 100 through its open gas discharge valve 20with displacement liquid being fed from the second pump 38 b to thefirst fill manifold 32 a and through its shuttle valve 14 which isopened to the first fill manifold 32 a.

A sixth tank bank, tank Tn, which is coupled to the second fill manifold32 a with its shuttle valve 15 rotated to a first opened position, isshown beginning its HPG evacuation cycle being fed displacement liquidfrom the first pump 38 a and first reservoir 36 a through the secondfill manifold 32 a and its open shuttle valve 15. The HPG is vented tothe HPM 100 through its open discharge valve 21. The designation of thefifth tank bank as tank Tn will be understood by one skilled in the artto indicate that the system is expandable beyond the number of tankbanks depicted in the figures.

In FIG. 6, the end phase of this sequence is shown with tanks T0 throughT3 emptied of displacement liquid and filled with low pressure gas withtheir valves 10, 11, 12, 13, 16, 17, 18 and 19 set in isolation modeisolating their low pressure gas contents. Tank T4, which has switchedto fluid drain mode, is shown in final drain mode with its gas dischargevalve 20 closed and its shuttle valve 14 opened to the first drainmanifold 30 a to drain displacement liquid into the first reservoir 36a. The sixth tank bank, tank Tn, is shown approaching the final phase ofdisplacing HPG into the HPM 100 through its open gas discharge valve 21with displacement liquid being fed from the first pump 38 a to thesecond fill manifold 32 b and through its shuttle valve 15 which isopened to the second fill manifold 32 b. For systems greater in sizethan that depicted in the figures, the second pump 38 b would feeddisplacement liquid to the next tank bank not shown in the figures orTable 1.

The above illustrates how through cross piping the initial batch ofdisplacement liquid from tank T0 can be transferred through all of thetank banks, alternating between different sets of tank banks andshuttling between the reservoirs. The sequence as described above cancontinue through many more tank banks or vessels within the desireddischarge time. The saving in storage space for displacement liquidresulting from this shuttle system is now useable for additional HPGstorage.

Multi port valving used in the forgoing description can also be replacedby single port valves according to prevailing design codes. TABLE 1Sequence of Displacement liquid Shuttle through Pumps & tanks tank T0tank T1 tank T2 tank T3 tank T4 Tank Tn FIG. 1 Gas Valve Closed ClosedClosed Closed Closed Closed Fluid Valve Closed Closed Closed ClosedClosed Closed Contents/ Disp Fluid HP Gas HP Gas HP Gas HP Gas HP GasStatus Stored Stored Stored Stored Stored Stored FIG. 2 Gas Valve ClosedOpen to HPM Open to HPM Closed Closed Closed Fluid Valve Open to Res R1Open to Open to Closed Closed Closed Pump P1 Pump P2 Contents/ DispFluid 30% HP Gas 70% HP Gas HP Gas HP Gas HP Gas Status Emptying toDisplacement Displacement Res R1 FIG. 3 Gas Valve Closed Closed Open toHPM Open to HPM Closed Closed Fluid Valve Closed Open to Open to Open toClosed Closed Res R2 Pump P2 Pump P1 Contents/ LP Gas/ 30% Disp Fluid30% HP Gas 70% HP Gas HP Gas HP Gas Status Liq Unloaded Emptying to R2Displacement Displacement Stored Stored FIG. 4 Gas Valve Closed ClosedClosed Open to HPM Open to HPM Closed Fluid Valve Closed Closed Open toOpen to Open to Closed Res R1 Pump P1 Pump P2 Contents/ LP Gas/ LP Gas/Disp Fluid 30% HP Gas 70% HP Gas HP Gas Status Liq Unloaded Liq UnloadedEmptying to R1 Displacement Displacement Stored FIG. 5 Gas Valve ClosedClosed Closed Closed Open to HPM Open to HPM Fluid Valve Closed ClosedClosed Open to Open to Open to Res R2 Pump P2 Pump P1 Contents/ LP Gas/LP Gas/ LP Gas/ 30% Disp Fluid 30% HP Gas 70% HP Gas Status Liq UnloadedLiq Unloaded Liq Unloaded Emptying to R2 Displacement Displacement FIG.6 Gas Valve Closed Closed Closed Closed Closed Open to HPM Fluid ValveClosed Closed Closed Closed Open to Open to Res R1 Pump P1 Contents/ LPGas/ LP Gas/ LP Gas/ LP Gas/ 30% Disp Fluid 30% HP Gas Status LiqUnloaded Liq Unloaded Liq Unloaded Liq Unloaded Emptying to R1Displacement

In the foregoing specification, the invention has been described withreference to specific embodiments thereof. It will, however, be evidentthat various modifications and changes may be made thereto withoutdeparting from the broader spirit and scope of the invention. Forexample, the reader is to understand that the specific ordering andcombination of process actions shown in the process flow diagramsdescribed herein is merely illustrative, unless otherwise stated, andthe invention can be performed using different or additional processactions, or a different combination or ordering of process actions. Asanother example, each feature of one embodiment can be mixed and matchedwith other features shown in other embodiments. Features and processesknown to those of ordinary skill may similarly be incorporated asdesired. Additionally and obviously, features may be added or subtractedas desired. Accordingly, the invention is not to be restricted except inlight of the attached claims and their equivalents.

1. A fluid storage and liquid displacement shuttle system comprising a plurality of tank banks each comprising a plurality of storage vessels, a discharge manifold coupled to a first end of the plurality of storage vessels, a first fluid shuttle circuit coupled at a first end to a second end of the plurality of storage vessels in a first set of the plurality of tank banks and at a second end to a second end of the plurality of vessels in a second set of the plurality of tank banks, and a second fluid shuttle circuit coupled at a first end to the second end of the plurality of storage vessels in the second set of the plurality of tank banks and at a second end to the second end of the plurality of vessels in the first set of the plurality of tank banks, wherein one of the plurality of tank banks includes a displacement liquid stored in its plurality of storage vessels and the other of the plurality of tank banks includes a fluid stored in the plurality of vessels intended to be evacuated from the plurality of vessels when displaced by the displacement liquid.
 2. The system of claim 1 further comprising a low pressure gas displacement system coupled to the plurality of vessels.
 3. The system of claim 1 wherein the first fluid shuttle circuit comprises a first drain manifold coupled in parallel to the plurality of vessels of the first set of the plurality of tank banks, a first reservoir coupled to the first drain manifold, a first pump coupled to the first reservoir, and a first fill manifold coupled to the first pump and to the plurality of vessels of the second set of the plurality of tank banks.
 4. The system of claim 3 wherein the first fluid shuttle circuit comprises a second drain manifold coupled in parallel to the plurality of vessels of the second set of the plurality of tank banks, a second reservoir coupled to the second drain manifold, a second pump coupled to the second reservoir, and a second fill manifold coupled to the second pump and to the plurality of vessels of the first set of the plurality of tank banks.
 5. The system of claim 4 wherein first and second check valves, respectively, interpose the first and second reservoirs and first and second pumps, respectively.
 6. The system of claim 1 wherein each of the plurality of vessels includes a multi-position shuttle valve positionable between a first position that fluidly isolates the vessel from the first and second fluid shuttle circuits, a second position that fluidly couples the vessel to the first fluid shuttle circuit, and a third position that fluidly couples the vessel to the second fluid shuttle circuit.
 7. The system of claim 6 wherein each of the plurality of vessels includes a multi-position discharge valve positionable between a first position that fluidly isolates the vessel from the discharge manifold and a second position that fluidly couples the vessel to the discharge manifold.
 8. The system of claim 1 wherein the stored fluid is a compressed gas.
 9. The system of claim 1 wherein the displacement liquid is a non reactive liquid.
 10. The system of claim 1 wherein the displacement liquid is natural gas liquid.
 11. The system of claim 1 wherein the plurality of storage vessels are vertically aligned relative to one another along axes extending from the first end to the second end of each of the plurality of storage vessels.
 12. The system of claim 1 wherein the plurality of storage vessel are coupled in parallel to the discharge manifold and the first and second fluid shuttle circuits.
 13. A method of storing and distributing displacement liquid suited for the purging stored fluids contained in storage vessels, comprising the steps of storing a displacement liquid in a first set of storage vessels, storing a fluid to be purged in a second and third set of storage vessels, shuttling the displacement liquid from the first set of storage vessels to the second set of storage vessel, displacing the stored fluid in the second set of storage vessels with the displacement fluid, and purging the stored fluid from the second set of storage vessels.
 14. The method of claim 13 further comprising the steps of shuttling the displacement liquid from the second set of storage vessels to the third set of storage vessel, displacing the stored fluid in the third set of storage vessels with the displacement fluid, and purging the stored fluid from the third set of storage vessels.
 15. The method of claim 14 wherein the step of shuttling displacement liquid to the second set of storage vessels includes draining the displacement liquid from the first set of storage vessels into a first shuttle circuit, and passing the displacement liquid through the first shuttle circuit to the second set of storage vessels.
 16. The method of claim 15 wherein the draining step includes feeding displacement liquid from the first set of vessels into a first manifold.
 17. The method of claim 16 wherein the passing step includes feeding displacement liquid from the first manifold into a first reservoir, pumping displacement liquid from the first reservoir into a second manifold, and feeding displacement liquid from the second manifold into the second set of storage vessels.
 18. The method of claim 17 wherein the step of purging the stored fluid from the second set of storage vessels includes exhausting gas from the second set of storage vessels.
 19. The method of claim 18 wherein the step of shuttling displacement liquid to the third set of storage vessels includes draining the displacement liquid from the second set of storage vessels into a second shuttle circuit, and passing the displacement liquid through the second shuttle circuit to the third set of storage vessels.
 20. The method of claim 19 wherein the draining step includes feeding displacement liquid from the second set of vessels into a third manifold.
 21. The method of claim 20 wherein the passing step includes feeding displacement liquid from the second manifold into a second reservoir, pumping displacement liquid from the second reservoir into a fourth manifold, and feeding displacement liquid from the fourth manifold into the third set of storage vessels.
 22. The method of claim 21 wherein the step of purging the stored fluid from the third set of storage vessels includes exhausting gas from the third set of storage vessels.
 23. A method of storing and distributing displacement liquid suited for the purging stored gas contained in storage vessels, comprising the steps of storing a displacement liquid in a first set of storage vessels, storing a gas to be purged in a plurality of gas storage vessels, shuttling the displacement liquid from the first set of storage vessels to alternating sets of the plurality of gas storage vessels, and exhausting gas from the plurality of gas storage vessels. 